Sole and Combined Application of Biodigestate, N, P, and K Fertilizers: Impacts on Soil Chemical Properties and Maize Performance

The fertilizing effects of biodigestate produced from biogas plants on crop and soil productivity are very scarce. Hence, a field study was conducted in 2022 at the Teaching and Research Farm of Bowen University, Iwo, Osun State, Nigeria. The study evaluated the effects of biodigestate fertilizer, applied alone or in combination with urea, single superphosphate, or muriate of potash fertilizers at low (N1, K1, and P1) and high (N2, P2, and K2) rates on soil chemical properties, growth, and yield of maize (Zea mays (L.)). The treatments were biodigestate alone (D), D + N fertilizer (urea) at 60 kg·ha−1 (DN1), D + N at 120 kg·ha−1 (DN2), D + P fertilizer (single superphosphate) at 30 kg·ha−1 (DP1), D + P at 60 kg·ha−1 (DP2), D + K fertilizer (muriate of potash) at 30 kg·ha−1 (DK1), D + K 60 kg·ha−1 (DK2), D + N1 + P1 + K1 (DN1P1K1), D + N2 + P2 + K2 (DN2P2K2) (10), and control. The 10 treatments were arranged in a randomized complete block design and replicated three times. Results showed that both low and high rates of fertilizer application improved soil chemical properties, growth parameters, and yield of maize compared with the control. High fertilizer rates (N2, P2, and K2) significantly enhanced soil chemical properties and growth parameters, but lower rates (N1, P1, and K1) resulted in higher maize yield. DN1 fertilizer significantly increased maize yield compared with DN2, DP1, DP2, DK1, and DK2. Overall, the treatment of DN1P1K1 demonstrated the highest grain yield, likely due to optimal nutrient supply from N, P, and K fertilizers, along with an improved soil environment facilitated by the biodigestate. The study recommends a balanced and sustainable fertilizer application strategy of 60 kg·N·ha−1, 30 kg·P2O5·ha−1, and 30 kg·K·ha−1 with 2500 L·ha−1 of biodigestate to enhance maize production while minimizing cost and environmental impact. However, for those aiming for maize fodder production, a higher fertilizer rate of 120 kg·N·ha−1, 60 kg·P2O5·ha−1, and 60 kg·K·ha−1 with 2500 L·ha−1 of biodigestate is advised.


Introduction
Te world population is estimated to reach 9 billion by 2050 [1,2]; therefore, there will be more food to feed the continuously growing population, especially in developing countries like Nigeria.Food stands as a paramount essential for humanity.To achieve the millennium development goal in food production, particularly in Nigeria, an abundant supply of food must be ensured.Te potential threat to food security due to dwindling soil fertility necessitates urgent attention.Several limitations signifcantly hinder the viability of agricultural sustainability in tropical soils.Tese include high bulk density, a diminished capacity to retain water and nutrients, and the soil's rapid mineralization of organic matter.As a result, the majority of soils are not conducive to achieving high crop yields due to their lightweight texture and inadequate nutrient reserves [3].Within tropical regions, sandy soils exhibit low levels of organic matter and essential nutrients such as nitrogen, phosphorus, potassium, calcium, and magnesium.
In addition, they possess a low cation exchange capacity (CEC) and have a limited capacity to store and provide soil moisture [4].For the foregoing reasons, farmers abandon the land for another after cultivating it for 2-3 years; however, due to the scarcity of land, there is a need to increase the yield per unit area of the existing cultivated lands.Enhancing soil productivity and increasing yield per unit area can be achieved through incorporating external inputs, such as organic and chemical fertilizers [5].
In this pursuit, the judicious application of fertilizers, including nitrogen (N), phosphorus (P), and potassium (K), has been a longstanding agricultural practice [6].Tese essential nutrients play critical roles in various physiological and biochemical processes vital for plant growth and development.However, conventional fertilizer use has raised concerns related to environmental sustainability, soil health, and the associated economic costs [5].
To address these concerns, there has been a burgeoning interest in exploring alternative fertilization approaches that mitigate environmental impact and optimize nutrient utilization efciency.One promising avenue is the use of biodigestate, organic matter derived from the anaerobic digestion of agricultural residues and organic waste, into the fertilization regimen.Just like organic manures, biodigestate not only provides essential nutrients but also enhances soil structure, microbial activity, and overall soil health [7].Te agricultural and horticultural sectors show signifcant promise for the utilization of biodigestate.It is commonly employed as a soil conditioner and organic fertilizer in these felds due to its abundance of nutrients and benefcial microorganisms for soil health [8,9].Te application of biodigestates is particularly benefcial for enhancing and modifying soil structures, improving soil nutrient levels, and fostering a variety of benefcial microorganisms with specifc functions, especially in soils that are marginal or depleted in nutrients when used as organic fertilizers [10].Notably, digestate serves as a slow-release fertilizer, supplying essential nutrients like nitrogen, phosphorus, and potassium (NPK), along with other crucial plant macronutrients necessary for the growth, health, and overall well-being of crop plants.Importantly, these positive efects are achieved without causing any harm to the soil [11,12].Currently, numerous studies are dedicated to exploring the impact of digestates on crop and vegetable yields as well as quality [13,14].
Terefore, when combined with conventional N, P, and K fertilizers, biodigestate has the potential to synergistically impact soil chemical properties and subsequently infuence crop performance.
Maize, scientifcally known as Zea mays (L.), stands out as a highly efcient and industrially signifcant cereal crop on a global scale.It holds the esteemed position of being the third most prominent cereal crop worldwide, following wheat and rice.Moreover, maize holds immense importance as a staple food in numerous countries.Its grains typically contain approximately 13% moisture, 10% crude protein, and impressive 70.3% carbohydrate content [15].As highlighted by Nweke [16], maize constitutes a signifcant portion, approximately 43%, of the caloric intake in an average Nigerian diet.Te fresh maize grains ofer culinary versatility; they can be roasted or cooked for immediate consumption.Alternatively, after undergoing the milling and boiling processes, the grains can be transformed into porridge.Maize shares a characteristic trait with other cereal crops; it is a nutrient-demanding plant, necessitating a rich nutrient supply for optimal growth and heightened productivity.
Attaining optimal maize yields necessitates a wellrounded and sufcient provision of nutrients, given that the reduction in soil fertility poses a signifcant hurdle for maize cultivation [17].Maize, like other cereal crops, is a heavy feeder and needs a high amount of nutrients for increased productivity [5].Te use of chemical fertilizer has been reported [5,18] to increase maize yields, but in Nigeria, its use is limited by high cost and scarcity during the time of its need (planting season) [18].Because of these, the use of organic fertilizers such as biodigestate was found to be useful in increasing crop production.Also, previous studies have shown that the appropriate use of biodigestate can increase soil nutrients, enhance crop nutritional quality, reduce greenhouse gas emissions, decrease crop disease, and have other benefts [19,20].However, biodigestate like other organic manures is limited by the slow-release nature of its nutrient [21], especially at the initial growth stage of maize.Terefore, to avert this problem, there is strong advocacy for integrating organic and inorganic fertilizers [22,23].
Several studies have been conducted on the integrated efect of organic and inorganic fertilizers on maize performance [24][25][26].It is important to conduct research to explore the appropriate N, P, and K levels with supplemental incorporation of biodigestate (organic fertilizer) for achieving optimum soil chemical properties and crop yield, as little overapplication of any of the fertilizers could be economical due to its cost.
In addition, this work investigates the sole and combined application of biodigestate, N, P, and K fertilizers, and their efects on soil chemical properties and maize performance.Te study seeks to contribute valuable insights that can inform sustainable agricultural practices, reduce the cost of fertilization, and promote optimal nutrient utilization and crop yields while safeguarding environmental and economic sustainability.

Site Description and Treatments.
A feld study was conducted in July 2022 at the Teaching and Research Farm of Bowen University, located in Iwo, Osun State, Nigeria (coordinates 7.6236 °N, 4.1890 °E, altitude 312 m above sea level).Te soil, identifed as sandy loam, formed from fnegrained granite gneiss and schist and is correlated with the Egbeda series (plinthic) as reported by Smyth and Montgomery [27] in their study of soils in Southwestern Nigeria.Te soil in this area is also similar to soils described as Plinthic Kandiudalf by Ajiboye et al. [28], following the classifcation of Soil Survey Staf [29].Te soil was formed in residuum, with an abrupt increase in clay content in the subsoil, and also possessed plinthic properties in the horizon 2 Te Scientifc World Journal within the frst 150 cm of the mineral horizons.Te pedogenesis of such soils formed from the basement complex in residuum has been described by Ande and Senjobi [30].
Te region experiences a bimodal pattern of rainfall, averaging around 1300 mm annually, and maintains an average temperature of 32 °C.Te climate is classifed as subhumid.

Land Preparation, Field Layout, Application of Biodigestate, and Sowing of Maize Seeds.
Te designated experimental area was ploughed and harrowed before delineating the feld layout promptly post-harrowing using ropes, pegs, and tape.Each experimental plot measured 12 × 3 meters and was subjected to a specifc treatment, replicated three times.Te plots were spaced at intervals of 0.5 meters, while blocks were separated by 1 meter.Te biodigestate used in this study was produced from the anaerobic codigestion of poultry droppings and piggery dung.A 20,000 l capacity digester currently used for the digestion of animal wastes accrued in the Bowen University Farm was used.It is a continuous digester with constant supply of poultry droppings and piggery dung, while the inoculum was from cow dung.In practice, the wastes are continuously digested in the digestion chamber, while the biodigestate slurry which is produced as a by-product after methane generation is channeled into a 1,000 l capacity holding chamber for collection.After collection of the slurry, it was analysed for various important parameters prior to the soil application.Te biodigestate was incorporated into the soils almost simultaneously with planting after the experiment layout on the feld at the rate of 2500 L•ha −1 , which is equivalent to 9 L plot −1 .A hand-held hoe was used to incorporate the biodigestate into the soil to a depth of approximately 20 cm.Hybrid maize from Seed Co, which is high yielding, was sown on the 8th of August 2022 at a depth of 2-3 cm.Tis variety is medium maturing and embedded with vitamin A. It can tolerate crowding and high population.Two seeds were planted in each hole with a spacing of 75 cm × 25 cm.Two weeks from the sowing date, the seedlings were thinned down to one plant per stand, resulting in a total of 192 plants per plot, approximately amounting to 53,333 plants per hectare.P fertilizer (single superphosphate 20% P 2 O 5 ) was applied at the rate of 30 kg•ha −1 and 60 kg•ha −1 , respectively, for P 1 and P 2 treatments at sowing, while K fertilizer (muriate of potash-60% K) was applied at 2 weeks after sowing (WAS) of maize.K fertilizer was applied at the rates of 30 kg•ha −1 and 60 kg•ha −1 , respectively, for K 1 and K 2 treatments.Urea fertilizer (46% N) was split-applied; the frst dose of 30 kg•ha −1 (N 1 treatments) and 60 kg•ha −1 (N 2 treatments) was made 3 WAS, while the remaining dose was applied 6 WAS.Urea and muriate of potash fertilizers were administered by side placement, positioned approximately 8-10 cm away from the seeds during the sowing process and at the base of the plant following germination at a depth of about 3 cm.Weeds were controlled with Paraforce (paraquat dichloride), a preplanting herbicide at a rate of 1.00 kg ai/ha and atrazine, a selective and systemic herbicide containing 80% atrazine WP at a rate of 1.5 kg ai/ha.Fall armyworm incidence was treated with caterpillar force (emamectin benzoate 5% WDG), a nonsystemic insecticide at a rate of 30 mL per 10 L of water two WAS and repeated at 4 WAS and the 6 WAS for efective control.Five plants were tagged from each plot for data collection.

Soil and Biodigestate
Analysis.Surface soil samples were randomly taken from the experimental feld at a depth of 0-15 cm for subsequent physical and chemical analyses before commencing the experiment.Te collected soil samples were air-dried, sieved through a 2-mm sieve, and preserved for analysis.Te sand, silt, and clay contents were determined using the hydrometer method [33].Soil pH was measured using a pH meter with a 1 : 2.5 soil/water ratio.Total nitrogen content was determined using the micro-Kjeldahl method [34], available phosphorus was assessed using the Bray 1 method [35], and calcium (Ca) and magnesium (Mg) were analysed using atomic absorption spectrophotometry (AAS).Potassium (K) and sodium (Na) levels were measured using fame emission photometry, following the procedure outlined by the Association of Ofcial Analytical Chemists [36].Organic carbon content was determined using the dichromate wet oxidation method according to Walkey and Black, as detailed by Nelson and Sommers [37].Upon completion of the experiment, soil samples were once again collected randomly from fve distinct locations within each plot from 0 to 15 cm soil depth.Tese samples were combined to create composite soil samples on a plot basis and subjected to the same abovementioned chemical analysis procedures.
A sample of the biodigestate was also collected for analysis of the following parameters: total nitrogen, available phosphorus, and exchangeable potassium, calcium, and magnesium, as described by Tel and Hagarty [38].

Determination of Growth and Yield Parameters.
In each plot, ten maize plants were designated for data collection purposes.Growth-related information was gathered at the tasseling stage of the maize plants.Te specifed Te Scientifc World Journal parameters encompassed plant height, leaf count, stem circumference, and leaf length.Plant height was measured with a tape measure, extending from the plant's base to its tassel.Leaf count was determined by simple enumeration.Te stem circumference was measured using a Vernier caliper.Leaf length was assessed using a tape measured from the sheath to the tip of the leaves.Te maize plants were allowed to undergo the drying process before the harvest.At the maize harvest (90 days after sowing), parameters pertaining to yield were recorded, including biomass weight, ear weight, cob weight, and shelled grain weight.
Biomass weight was measured by cutting and weighing the entire maize plant (including stalks, leaves, ears, and cobs).Ear weight was deduced by detaching the ears from the harvested plants and weighing them.Shelled grain weight was determined by shelling the kernels from the cobs and weighing them.

Data Analysis.
Te data collected for growth and yield parameters underwent statistical analysis through Analysis of Variance (ANOVA) using the Statistical Analysis System (SAS) [39], version 9.4.Mean values were then distinguished through Tukey pairwise comparisons at a signifcance level of p < 0.05.

Initial Soil Characteristics of the Site Used.
Te initial soil characteristics of the site used for the experiment in 2022 are presented in Table 1.Te experimental site was slightly alkaline and sandy loam in texture.Te site was low in organic matter (OM), N, and P and slightly acidic.Te exchangeable bases K, Ca, and Mg were adequate according to the critical levels of 3.0% OM, 0.2.0%N, 10.0 mg•kg −1 P, 0.16-0.20 −1 K, 2.0 cmol•kg −1 Ca, and 0.40 cmol•kg −1 Mg recommended for crop production in the agroecological zone in Nigeria [40].Chemical analysis of biodigest used (Table 2) indicates that it contains nutrient elements (N, P, K, Ca, and Mg) required for the growth of a cereal crop such as maize.

Efects of Biodigestate, N, P, and K Fertilizers on Soil
Chemical Characteristics.Te results of the efects of biodigestate, N, P, and K fertilizers on soil chemical characteristics are presented in Table 3. Application of biodigestate alone (D) or in combination with N, P, and K fertilizers at any rate increased the pH, OC, N, P, K, Ca, Na, and Mg content of the soil relative to the control.Te high rate of fertilizer (N 2 , P 2 , and K 2 ) increased soil chemical properties relative to low rates (N 1 , K 1 , and P 1 ).DN 2 P 2 K 2 increased soil chemical properties relative to DN 1 P 1 K 1 .Te integration of D with either N, P, or K fertilizer or their combination N + P + K (DN 1 P 1 K 1 and DN 2 P 2 K 2 ) increased soil chemical properties relative to D alone.In relative terms, among the combination of D with N, P, and K, DN 2 increased soil chemical properties the most.

Efects of Biodigestate, N, P, and K Fertilizers on the Growth
and Yield of Maize.Tables 4 and 5, respectively, show the result of the efect of biodigestate, N, P, and K fertilizers on some growth (plant height, number of leaves, leaf length, and stem girth) and yield (biomass weight, ear weight, cob weight, and grain weight) parameters of maize.Application of biodigestate alone (D) or in combination with N, P, and K fertilizers at any rate increased the growth and yield parameters of maize relative to the control.Tere were no signifcant diferences between biodigestate alone (D) or in combination with N, P, and K fertilizers and the control treatment for a number of leaves/plants (Table 4).Te high rate of fertilizer (N 2 , P 2 , and K 2 ) increased the growth parameters of maize relative to low rates (N 1 , K 1 , and P 1 ).DN 2 P 2 K 2 increased maize growth compared with DN 1 P 1 K 1 (Table 4).Te integration of D with either N, P, or K fertilizer or their combination N + P + K (DN 1 P 1 K 1 and DN 2 P 2 K 2 ) increased maize growth relative to D alone.Te low rate of fertilizer (N 1 , P 1 , and K 1 ) increased the yield parameters of maize relative to high rates (N 2 , K 2 , and P 2 ).DN 1 P 1 K 1 increased maize yield relative to DN 2 P 2 K 2 (Table 5).In relative terms, among the combinations of D with N, P, and K, DN 1 increased the yield of maize the most.Relative to DN 2 , DP 1 , DP 2 , DK 1 , and DK 2 , DN 1 increased grain weight of maize by 65.4, 56.0, 147.0, 8.98, and 24.1%, respectively.DN 1 P 1 K 1 increased maize yield compared with DN 2 P 2 K 2 .Overall, DN 1 P 1 K 1 has the highest grain yield.

Discussion
Te experimental site's soil exhibited low nutrient levels, particularly in terms of soil organic carbon (OC), nitrogen (N), and phosphorus (P).Tese soil conditions are typical of  1) show that the site was low in OC, N, P, K, Ca, and Mg.Tese conditions are the characteristics of Alfsols in southwest Nigeria [41,42].Adekiya et al. [43] and Agbede et al. [3] earlier found that tropical soils are low in soil fertility.Te inadequate levels of total N and available P before planting were attributed to the low organic matter in the soil.Agbede et al. [3] also attributed the low N and P on preplanting soil to low organic matter.
Te use of biodigestate alone (D) or in combination with N, P, and K fertilizers at various rates led to an improvement in soil characteristics, including an increase in pH, organic carbon (OC), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sodium (Na), and magnesium (Mg) content compared to the control.Tis outcome was anticipated due to the initially low soil fertility, as indicated in (Table 1).Urea fertilizer, muriate of potash (potassium chloride), and single superphosphate are commonly used in agriculture to provide essential nutrients to plants and improve soil fertility.Each of these fertilizers has specifc efects on soil properties and nutrient content, which can infuence soil pH and nutrient levels relative to a control (soil without fertilizer).Biodigestate also contains nutrients (Table 2), and these nutrients are released into the soil and used by plants.Te increase in soil content of basic macronutrients was also observed in other studies [44,45], which can be attributed to   Values followed by similar letters under the same column are not signifcantly diferent at p � 0.05 according to Duncan's multiple range test.
Te Scientifc World Journal their high content in the organic material used.In an experiment to assess the fertilizing efects of digestate on chemical and biological soil properties in a feld experiment in eastern Portugal with two horticultural crops [46], in addition to N, digestate supplied signifcant amounts of P, Ca, K, and Mg and signifcantly increased soil Olsen P, mineral N, and organic C. It was reported that the application of biogas digestate signifcantly increased soil pH, content of organic carbon, total N, and available forms of P, K, and Mg [47,48].According to Smith et al. [49], biogas digestate has great potential to increase soil C sequestration.It increased soil OC because the biodigestate was prepared from organic waste (cow dung).Tis result is not in line with the result of Muhammad [50] which reported that the application of bio-slurry in liquid and composted form brought no change in the organic matter content of the soil.Te rise in soil organic matter and nutrient levels, stemming from the usage of urea, muriate of potash, and single superphosphate fertilizers, might be attributed to the accelerated and robust growth of maize plants following their application.In addition, the deposition of crop residues, particularly leaves, during plant maturity could have contributed organic matter to the soil.Tis collective input has led to modest augmentation in the soil's organic matter, as well as its nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) content [22].Debebe and Itana [51] also revealed in their study that bio-slurry increases organic carbon, phosphorus, and cation exchange capacity of soil.Alemneh [52] also reported an increase in soil chemical properties due to bio-slurry application.
Te high rate of fertilizer (N 2 , P 2 , and K 2 ) and DN 2 P 2 K 2 increased soil chemical properties relative to low rates (N 1 , K 1 , and P 1 ) and DN 1 P 1 K 1 due to higher concentrations of nutrients in higher rates relative to lower rates.
Te integration of D with either N, P, or K fertilizer or their combination N + P + K (DN 1 P 1 K 1 and DN 2 P 2 K 2 ) increased soil chemical properties relative to D alone.Tis was due to the high quantity of N, P, and K in urea, single superphosphate, and muriate of potash fertilizer, respectively, compared with D. Biodigestate (D) contains a diverse range of nutrients due to its organic origin, including nitrogen, phosphorus, potassium, and various micronutrients.Combining this with urea, muriate of potash, and single superphosphate, which are concentrated N, K, and P sources, ofers a more comprehensive and balanced nutrient supply to the soil, enriching its chemical properties.
Biodigestate (D) increased the growth and yield of maize in this study compared with the control.Tis was due to the high concentration of nutrients in D and subsequent nutrient release.In a comparative experiment aiming to assess the impact of bio-slurry and synthetic fertilizer on soil characteristics, growth, and yield of white cabbage [51], it was observed that the application of bio-slurry led to an increase in cabbage yield compared to the control group.
Te enhanced growth and yield of maize observed after the application of N, P, and K fertilizers can be attributed to the defciency of vital nutrients in the soil at the experimental site, crucial for the growth and yield of maize.Ogunboye et al. [18] and Amali and Namo [53] reported an increase in maize yield due to urea fertilizer.Ahmad et al. [54] and Ademba et al. [55] reported an increase in maize yield due to phosphate fertilizer.Ul-Allah et al. [56] also recorded a signifcant increase in the yield of maize due to potassium fertilizer.Te reason for these responses was that nitrogen present in urea fertilizer stimulates leaf growth and is essential for protein and chlorophyll formation.On the other hand, phosphorus in superphosphate aids in root development, energy transfer reactions, and cell division and multiplication.In addition, potassium supports stem development, cell division, and the formation and movement of carbohydrates from source to sink.
Te integration of D with either N, P, or K fertilizer or their combination N + P + K (DN 1 P 1 K 1 and DN 2 P 2 K 2 ) increased maize growth relative to D alone.Te elemental content of nitrogen (N), phosphorus (P), and potassium (K) in urea, muriate of potash, and single superphosphate fertilizers exceeded that found in biodigestate.However, upon addition to these fertilizers, biodigestate, being of organic origin, contributes organic matter to the soil.Tis organic matter enhances soil structure, augments water retention, and boosts the microbial activity.In addition, this organic matter serves as a gradual-release nutrient source, providing consistent nourishment to maize.Te humus generated by the biodigestate aids in retaining nutrients released from swiftly mineralized chemical fertilizers within the rooting zone [57], preventing leaching commonly observed in tropical soils.Consequently, these fosters improved nutrient uptake efciency and heightened yields compared to using biodigestate alone or employing no additives.Te presence of biodigestate may have also aided in maintaining a higher amount of applied urea in the soil, either in its original form or as ammonium ions, for an extended duration.Tis, in turn, resulted in enhanced nitrogen uptake efciency [58,59].Debebe and Itana [51] similarly noted a signifcant increase in cabbage yield when bio-slurry was combined with chemical fertilizer as opposed to using bio-slurry in isolation.Tese fndings align with the research of Dinka et al. [60] and Naiji and Souri [61], which indicate that an integrated nutrient approach led to higher crop yields compared to relying solely on recommended inorganic or organic fertilizers.Te authors further emphasized that such integration of organic and inorganic fertilizers has the potential to enhance soil productivity and quality over the long term.
Te high rate of fertilizer (N 2 , P 2 and K 2 ) increased the growth parameters of maize relative to low rates (N 1 , K 1 , and P 1 ), whereas higher yield was recorded at the low rate (N 1 , K 1 , and P 1 ) relative to the high rate of fertilizer (N 2 , P 2 and K 2 ).Tis result could be because of higher nutrient concentrations at the (N 2 , P 2 and K 2 ) level (Table 3), and the maize plant might have a focus on vegetative development, such as leafy biomass, at the expense of reproductive structures such as ears and kernels, leading to lower yields.Conversely, a maize plant with lower growth (N 1 , K 1 , and P 1 ) might allocate more resources towards reproductive structures, resulting in a higher yield despite limited vegetative growth.Brewbaker [62] found that as nutrient levels Te Scientifc World Journal increase, the overall biomass yield of maize plants will continue to rise, primarily due to plant enlargement rather than an increase in grain yield.Before now, White [63] similarly noted that elevated levels of nitrogen may slightly enhance the maize yield, but the cost-efectiveness of this improvement is questionable.In addition, higher phosphorus rates did not elicit any noticeable response in yield in maize.
DN 1 increased the yield of maize relative to DN 2 , DP 1 , DP 2 , DK 1 , and DK 2 .Tese yield responses to fertilization show that N defciency was the most limiting condition in maize production in the area.Although, DN 1 was able to be adjudged the best and surpassed DK 1 probably due to the higher initial K content of the soil (Table 1).Apart from N, K is another important nutrient necessary for maize cultivation [64].Nitrogen promotes the growth of leaves, stems, and overall vegetative parts of the maize plant.It is essential for lush, green foliage and robust plant structure, whereas K is important among other things in supporting the flling of maize kernels, leading to higher grain weight and improved quality.It is particularly important during the grain-flling stage.Under North American conditions, a maize crop producing 9.5 tonnes of grain per hectare can remove 191 kg N•ha −1 , 89 kg P 2 O 5 •ha −1 , and 235 kg K 2 O ha −1 [64].
Overall, DN 1 P 1 K 1 has the highest grain yield, which could be a result of optimum nutrient supply from N, P, and K fertilizers and better physical soil environment created by the biodigestate.

Conclusion
Biodigestate (D) fertilizer applied alone or in combination with sole urea (N), single superphosphate (P), or muriate of potash (K) fertilizer or their combinations (N + P + K) at low (N 1 , K 1 , and P 1 ) and high (N 2 , P 2 , and K 2 ) rates improved soil chemical properties, growth, and yield of maize compared with the control.Te integration of D with either N, P, or K fertilizer or their combination N + P + K (DN 1 P 1 K 1 and DN 2 P 2 K 2 ) increased soil chemical properties relative to D alone.In relative terms, among the combination of D with N, P, and K, DN 2 increased soil chemical properties the most.
Te high rate of fertilizer (N 2 , P 2 and K 2 ) increased soil chemical properties and growth parameters of maize relative to low rates (N 1 , K 1 , and P 1 ), whereas higher yield was recorded at the low rate (N 1 , K 1 and P 1 ) relative to the high rate of fertilizer (N 2 , P 2 , and K 2 ).Tese results could be because of higher nutrient concentrations at the (N 2 , P 2 , and K 2 ) level which might have made the maize plant focus on vegetative development, such as leafy biomass, at the expense of reproductive structures such as ears and kernels, leading to lower yields.DN 1 increased the yield of maize relative to DN 2 , DP 1 , DP 2 , DK 1 , and DK 2 .Overall, DN 1 P 1 K 1 has the highest grain yield, which could be as a result of optimum nutrient supply from N, P, and K fertilizers and better physical soil environment created by the biodigestate.Terefore, to avoid waste of fertilizer due to cost and negative environmental efect excessive fertilization, the lower rate of N (60 kg N ha −1 ), P (30 kg P 2 O 5 ha −1 ), and K 30 kg K ha −1 fertilizers with 2500 L•ha −1 of biodigestate is recommended for sustainable maize production; however, for those who want to use maize as fodder, a higher rate of N (120 kg N ha −1 ), P (60 kg P 2 O 5 ha −1 ), and K 60 kg K ha −1 fertilizers with 2500 L•ha −1 of biodigestate is recommended.

Table 1 :
Initial soil characteristics of the experimental site before maize sowing.

Table 2 :
Nutrient values of the biodigestate used.

Table 4 :
Growth parameter of maize after application treatments.
Values followed by similar letters under the same column are not signifcantly diferent at p � 0.05 according to Duncan's multiple range test.

Table 5 :
Yield parameter of maize after application treatments.